Do some materials appear to be darker at grazing angles ?
It's not about being lighter or darker. It's that the material is more reflective at grazing angles. So if the surrounding environment is lighter than the object, the grazing angles will appear lighter. If the surrounding environment is darker it will appear darker. The material's lowest point of reflectivity is also a factor.

To understand why the material reflects more at grazing angles, take this example of a sphere with lots of tiny particles being emitted from it. These little shards reflect light more, whereas the sphere itself absorbs it more (as perceived from the viewing angle). This is a simulation of how roughness works. The "roughness" value of a shader simulates the effect of microscopic bumps on the surface of materials.



In the image below, the sky is a black and white gradient. Here we can see that the Fresnel effect is not lightening or darkening, but rather reflecting more or less. So areas of the sky that are more white are causing lighter edges and areas that are more black are causing darker ones.